1. Field of the Invention
The present invention relates to a microscope apparatus used for an observation or examination of a specimen and to a focal-depth enlarged image generation method used therefor.
2. Description of the Related Art
Microscopes are widely used for defect inspection and structural observation of a device pattern in the processes for producing a semiconductor, et cetera. Associated with rapid progress in further miniaturization and complexity of device patterns in recent years, there have been increased demands on the optical microscopes for further improvement in resolution. In order to meet such demands, the use of an object lens with a large numerical aperture NA and the use of light with shorter wavelengths, such as infrared, for an illumination light have been tried.
Enlarging the NA of an object lens or shortening the wavelength of an illumination light causes a focal depth to become shallower. In such a case, if there is a step that is larger than the depth of focus in the specimen, a blurred image will result. Therefore, it is desired that the microscopes obtain a high resolution, large focal depth image.
Meanwhile, the materials used for producing a semiconductor, such as aluminum (Al) and copper (Cu), possess different reflectivities and absorptivities against infrared with different wavelength bands so that a defect analysis and a structural analysis that take advantage of these characteristics can be performed on an infrared microscope. In a specific example, a semiconductor device is observed by changing the wavelength band of an infrared light used as the illumination and a change in the reflectivities of a device material is detected by a change in the pixel value of an observation image, and thereby the device material can be identified. Such a microscope using infrared lights with a plurality of wavelength bands is called a plural wavelengths infrared microscope.
The depth of focus is different depending on the wavelength in the plural wavelengths infrared microscope and therefore there is a possibility that an observation image picked up with a long wavelength band will be in focus, while a part of the observation image picked up with a short wavelength may be out of focus. Therefore, a depth of focus needs to be increased for a shorter wavelength band.
There have conventionally been various concepts as methods for increasing a depth of focus. For example, reference patent document 1 has disclosed a technique that adds together plural pieces of images (the resultant image is noted as “addition image” hereinafter) with different positions respectively in focus in the optical axis direction and applies a restoration process to the addition image using a restoration filter, thereby restoring one piece of a focused image.
A conceivable method for obtaining an addition image is to change the focus positions continuously and to also accumulate the images imaged on the photo acceptance unit of an imaging element. In this method, the input and addition of images with their focuses being changed continuously are performed in the imaging element by utilizing an accumulation effect of a photo energy of the imaging element.
In the meantime, the above described method of continuously changing the focus position and obtaining an addition image utilizing the accumulation effect of the imaging element is faced with a limitation in the dynamic range of a camera in which an imaging element is incorporated and therefore an addition image with saturated brightness tends to be produced. In order to prevent the occurrence of such saturated brightness, it is conceivable to reduce the exposure time of the camera or the irradiation light volume of a light source; either case, however, is faced with the problem of a degraded S/N ratio in the addition image.
In the method of obtaining an addition image according to patent document 1, there is a possibility that saturated brightness will occur in the high reflectance part of the addition image if the exposure time of a camera and the emission light volume of a light source are set in line with the low reflectance part of a specimen.
Patent document 1: Registered Japanese Patent No. 3191928